Geological Summary for New Zealand Area, June 2006

Earthquake activity continues at a low level. Mt Ruapehu, White Island, Raoul Island continue at Alert Level 1. Change at Mt Ngauruhoe.

Geonet, the USGS (NEIC) and IGNS reported 33 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during June 2006. The magnitude distributions were as follows:
M6 to 6.9 (none), M5 to 5.9 (2), M4 to 4.9 (17) M3 to 3.9 (11).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.

Earthquake activity deep under the North island, which eased during May, increased again during June. Ten quakes with magnitudes between 3.8 and 5.6 occurred at depths between 120 and 255 km during June. The largest event was a magnitude 5.6 earthquake on the evening of June 15th, 170 km deep, located 30 km north-west of Taupo. This quake was felt along the East Coast and in the lower half of the North Island, according to Hazardwatch.

Yet another cluster of earthquakes at widely varied depths struck near Milford Sound. A deep magnitude 4.1 event occurred on June 6th, a magnitude 4.7 quake occurred at 12 km depth on the 19th and a magnitude 4.0 event occurred at a depth of 5 km on the 28th. The earthquake on the 28th was described by Hazardwatch as being the largest of a series of quakes recorded in the area over a period of several days.

A cluster of three earthquakes with magnitudes between 3.5 and 4.6 at depths between 15 and 20 km struck at locations near Pongaroa (Tararua District) and Porangahau (Southern Hawke’s Bay) between the 5th and 20th.

A pair of magnitude 4.0 quakes struck about 5 hours apart at a location 20 km east of Seddon, Marlborough on June 16th. They were both shallow events, at depths of 8 and 11 km, and were felt in various suburbs of Wellington.

A swarm of light very shallow earthquakes were felt on the northern side of Lake Taupo on June 18th. The first sequence of about 20 events occurred up until 10:58 a.m. when the largest event of magnitude 2.7 was felt at Wairakei and Acacia Bay, Taupo. A magnitude 2.2 quake at 12:18 was slightly deeper at 4km, and a magnitude 2.4 event at 1:53 p.m. was the shallowest at 1 km depth.

Vulcanologists reported the nation’s volcanoes to be quiet during June. Their status can be summarised as follows:
Mt Ruapehu (Alert Level 1).
White Island (Alert Level 1).
Raoul Island (Alert Level 1).
Mt Ngauruhoe (changed to Alert Level 1)

Mt Ruapehu’s crater lake level rose to about 1.5 metres above the tephra dam as its temperature climbed to 32 °C during the first half of the month. The lake level then dropped slightly and the temperature declined to 24 °C. Weak volcanic tremor was recorded early in the month, with a few volcano-tectonic and one volcanic earthquake being recorded in the week to the 23rd after which seismic activity declined to low levels.

Seismic and geothermal activity at White Island were described as “very quiet” by Hazardwatch with the lake level stable, and the lake temperature falling through 53 °C to 50 °C where it remained at month’s end.

Raoul Island was also quiet during June, with the level of Green Lake continuing to fall slowly, but still above its pre-eruption level. Hazardwatch mentioned that nearby earthquake activity was tectonic rather than volcanic.

GNS Science issued three alert bulletins for Mt Ngauruhoe during June, and raised the volcano’s alert status to 1 on the 6th. The number of shallow low frequency earthquakes near Ngauruhoe increased mid-May, prompting further investigation. Additional monitoring equipment was deployed and increased sampling of vents was undertaken after the number of earthquakes increased to about 50 per day during the first week of June. Earthquake rates dropped to about 10-20 per day by mid-month, with the increased surveillance placing the activity at depths of 1 – 4 km slightly to the north-east of the summit with earthquake magnitudes of about 1. Fumarole temperatures were at similar levels to April 2006 values, and no significant carbon dioxide gas release was occurring through the soils when readings were compared with those made in May 2003.

Earthquake rates increased again to about 10-40 per day by the 23rd, but an attempt to retrieve portable seismic instruments from Ngauruhoe was thwarted by the weather. One instrument was retrieved by the end of the month.

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